1. Field of the Invention
The invention comprises a control for a vehicle powertrain having an internal combustion engine wherein minimum engine load is tightly controlled.
2. Background Art
Parallel-series hybrid electric vehicle powertrains capable of embodying the present invention are disclosed in U.S. patent application Ser. No. 10/709,537, filed May 12, 2004, now U.S. Pat. No. 7,013,213, and U.S. patent application Ser. No. 10/905,324, filed Dec. 28, 2004, now U.S. Pat. No. 7,223,201. Each of these patent applications is assigned to the assignee of the present invention.
Parallel-series hybrid electric vehicle powertrains provide power flow paths to vehicle traction wheels through gearing. In one operating mode, a combination of an internal combustion engine and an electric motor-generator subsystem may define in part separate torque delivery paths. The motor-generator subsystem includes a battery, which acts as an energy storing medium. In a first forward driving mode, the engine propels the vehicle using reaction torque of a generator, which is a part of the motor-generator subsystem. Planetary gearing makes it possible for the engine speed to be controlled independently of vehicle speed using generator speed control. In this configuration, engine power is divided between a mechanical power flow path and an electrical power flow path. Electrical power is distributed from the engine to the generator through transmission gearing. The generator is electrically coupled to an electric motor of the motor-generator subsystem, which in turn drives the vehicle traction wheels. Because the engine speed is decoupled from the vehicle speed, the powertrain emulates the characteristics of a continuously variable transmission during a driving mode in which the engine is active.
The electric motor provides a braking torque to capture vehicle kinetic energy during braking, thus charging the battery as the motor acts as a generator. Further, the generator, using battery power, can drive against a one-way clutch on the engine power output shaft to propel a vehicle in a forward drive mode as the generator acts as a motor.
As in the case of conventional continuously variable transmissions in vehicle powertrains, it is possible to achieve better fuel economy and exhaust gas emission quality by operating the engine at or near the most efficient operating region of its engine speed and torque relationship. The engine can be stopped if the engine operating conditions are not favorable for high fuel efficiency operation or if the engine is not in a high emission quality control region. In this way, the two power sources (i.e., the engine and the motor-generator subsystem) can be integrated and coordinated to work together seamlessly to achieve the goal of better fuel economy and emissions control.
A vehicle system controller performs the coordination of the control of the two power sources. Under normal powertrain operating conditions, the vehicle system controller interprets a driver demand for acceleration or deceleration and then determines when and how much torque each power source needs to provide in order to meet the driver's demand and achieve specified vehicle performance. Specifically, the vehicle system controller determines the speed and torque operating point for the engine.
In a hybrid powertrain of this kind, there are many operating conditions that require the engine to be operated near its minimum load or minimum torque limit. If the engine has an electronic throttle control, as distinct from a mechanically controlled throttle valve, the engine throttle element is controlled by a torque-based algorithm, which can result in a variation in the minimum engine load from a pre-calibrated value. If the minimum engine load is not tightly controlled, it is possible that a misfire condition will occur if the load is too low. Further, the hybrid powertrain battery might be overcharged if the minimum engine load is not tightly controlled and if the engine load is too high.